System for correcting the shift of an electron-gun beam from the desired region of impingement



April 5, 1966 T. K. CAULEY SYSTEM FOR CORRECTING THE SHIFT OF ANELECTRON-GUN BEAM FROM THE DESIRED REGION OF IMPINGEMENT Filed July 19,1963 2 Sheets-Sheet l IN VE/V TOR. THOMAS K. CAULEY WK? w;

Attorney April 5, 1966 T. K. CAULEY 3,244,855

SYSTEM FOR CORRECTING THE SHIFT OF AN ELECTRON-GUN BEAM FROM THE DESIREDREGION OF IMPINGEMENT Filed July 19, 1963 Y 2 Sheets-Sheet 2 INVENTORTHOMAS K. CAULEY A Harney United States Patent 3,244,855 SYSTEM FORCORRECTING THE SHIFT OF AN ELECTRON-GUN BEAM FROM THE DESIRED REGION OFIMPINGEMENT Thomas K. Cauley, West Miiflin Borough, Pa., assiguor toUnited States Steel Corporation, a corporation of New Jersey Filed July19, 1963, Ser. No. 296,320 1 Claim. (Cl. 219-121) This invention relatesto a system for correcting the tendency of a beam of electrons from aso-called electron gun, to wander from the desired area of impingement.

Electron-beam guns have found extensive use in indus try for surfaceheating by bombardment. One instance is the art of coating with metal byvapor deposition in a vacuum, where the beam from the gun is directedonto the surface of a mass of coating metal to vaporize it. In thecontinuous coating of strip by this method, the crucible used to holdthe coating metal, e.g., aluminum, is disposed transversely of the pathof the strip in order to cover the entire strip width. This means thatthe crucible must be narrow or the mass of coating metal will beexcessive.

In heating the surface of an elongated narrow crucible by electronbombardment, it is important to keep the electron beam constantly inclose alinement with the center line of the crucible, both to assureefficient heating and to prevent damage to the crucible. The source ofthe beam is usually a filamentary cathode and the beam is therefore widein the direction of the cathode length but is concentrated by shieldingto a small dimension in the direction normal to the cathode. The sourceis usually on one side of the crucible and the beam is bent by atransverse magnetic field or an accelerating voltage so it will curvedownwardly onto the metal in the crucible, as shown in Simons, Jr.,United States Patent No. 3,046,936.

The object of my invention is to provide a system for controlling themagnetic field or the accelerating voltage so that the curvatureimparted to the beam will be such as to cause impingement of electronsprincipally along the center line of the crucible and the mass of metalcontained in the recess or cavity therein. In a preferred embodiment, Iemploy elongated sensing electrodes extending along each side of thecrucible cavity so they will be subject to the impingence of electronsshould the beam wander from the desired condition of alinement with thecavity. I provide means responsive to the resulting current in thesensing electrodes for varying the magnetic field or acceleratingpotential so as to increase or decrease the curvature of the beam tobring its area of impingence back to its normal position along thecenter line of the mass of metal in the crucible.

A complete understanding of the invention may be obtained from thefollowing detailed description and explanation which refer to theaccompanying drawings illustrating the present preferred embodiment. Inthe drawings:

FIGURE 1 is a diagrammatic showing of the invention incorporating aknown arrangement of crucible and an electron gun for heating the metaltherein; and

FIGURES 2 and 3 are similar diagrams showing modifications.

Referring now in detail to the drawings and, for the moment, to FIGURE Iparticularly, an elongated crucible of refractory material, such asboron nitride, has a recess or cavity in its top face adapted to containa charge of metal 11, such as aluminum, to be vaporized and deposited ona strip (not shown) passing above the crucible in a direction normalthereto. An electron gun 12 extends along the crucible on one sidethereof. It comice prises a tungsten-filament cathode 13 extendingbetween suitable supports (not shown), a focussing electrode 14 and agrounded accelerating anode 15.

A transformer 16 furnishes heating current to cathode 13. Crucible 10 isgrounded as is also the anode 15. Cathode 13 is maintained at a negativevoltage of, say 10,000 volts, and electrode 14 at a somewhat greaternegative voltage by connections as shown to a direct-current source 19.The metal charge 11 in crucible 10 is the main anode toward whichelectrons from the cathode flow. The curvature of their paths indicatedat 13a depends on the voltage applied to cathode 13 and the intensity ofa magnetic field indicated at 17 between the poles of an electromagnet18. The apparatus described above is conventional and is disposed withina suitable enclosure (not shown) evacuated to a high degree, i.e., to apressure less than 30 microns.

In carrying out my invention, I provide crucible 10 with parallelsensing electrodes 20 and 21 extending therealong, one on each side ofthe recess or cavity therein containing charge 11. In the event ofdisplacement of electron beam for any reason, from the desired area ofimpingence on charge 11, indicated at 22, electrons will collect on oneor the other of electrodes 20 and 21. These electrodes are connected tothe control windings 23 and 24, respectively, of a magnetic amplifier 25of known type such as the Norbatrol Electronic Corporations Linear Powercontroller model 104C101, having a bias winding 26 thereon connected toa direct-current source 27. The output of amplifier 25, as modulated bycurrent from electrodes 20 and 21 through windings 23 and 24, is appliedto the winding of electromagnet 18.

A bias current applied to winding 26 of amplifier 25 is provided bysource 27 under the control of an adjustable bias resistor 28 to set thenorminal value of magnetizing-current output from the amplifier.Resistor 28 is adjusted so that the electron beam impinges on charge 11centrally between the electrodes 20 and 21 when the control windings 23and 24 are carrying equal currents. If, for any reason, the beam shouldwander toward one of the electrodes 20, 21, the resulting current insuch electrode will produce a control current adding to or subtractingfrom the nominal value of the magnetizing current and move the beam awayfrom that sensing electrode and toward the other. In this manner, themagnetic field is controlled so as to center and aline the electron beamwith area 22..

The embodiment of FIGURE 1 uses a single electron gun and magneticfield. For long crucibles, it may be necessary to use several gunsdisposed end-to-end along the crucible length. With this arrangement,the several electron beams cannot be individually regulated by varyingthe intensity of the magnetic field, because this would affect theelectron beams from all the guns. Effective control can be obtained,however, by regulating the accelerating potential of each gun on thebasis of signals indicating the extent and direction of the wandering ofits electron beam from the desired area of impingence. An arrangementsuitable for this purpose is shown in FIG- URE 2. For purposes ofdescription, I have shown therein two magnetic amplifiers 25, each onecontrolling one of the high-voltage sources 19 to vary the acceleratingeffects of the cathodes 13 of their electron-beam guns 12 and 12a,respectively.

It may be noted that the sensing electrodes 20 and 21 will be subjectedto vapor deposition from charge 11 as well as intense heating.Preferably, therefore, they should be made from a refractory metal, suchas tungsten, and supported in any convenient manner to prevent theirsagging.

Another embodiment of my invention is shown in FIG- URE 3 includingmeans for establishing two low-current electron beams 29 and 30 betweencathodes 31 and anodes 32. These beams replace the tungsten electrodes20 and 21. Electrons are emitted from hot cathodes 31 and areaccelerated by suitable voltage sources 33, along paths 29 and 30parallel to the crucible, to anodes 32. The line of travel of each ofthese secondary electron beams is located substantially where one of thesensing electrodes 20 and 21 would be placed. When the main beam 13a ofcharge-heating electrons is diverted by any means to cross the path ofthe secondary electron beams, the space charge which the main beamproduces in the path of the secondary beam, lowers the current carriedin the latter. Since this current passes through one of the controlwindings of a magnetic amplifier 25 in the manner previously described,the electromagnetic field, created by magnet 18 for bending the mainbeam, is altered to force it back toward the center line of crucible 10.

It will be evident that my invention affords simple yet effective meansfor preventing wandering of the area of impingence of an electron beamfrom the desired location, thereby improving the eificiency ofvaporizing coating metal and protecting the crucible from excessiveheating.

Although I have disclosed herein the preferred embodiment of myinvention, I intend to cover as well any change or modification thereinwhich may be made without departing from the spirit and scope of theinvention.

I claim:

In the combination with an elongated crucible containing a charge ofmetal to be vaporized, of an electron-beam gun extending alongside saidcrucible adapted to deliver an arcuate beam of electrons onto the top ofsaid charge and means maintaining the curvature of the paths ofelectrons making up said beam, the improvement comprising meansresponsive to a shift in the area of impingence of said beam on saidcharge and means controlled by said last-mentioned means forcompensatorily adjusting said curvature-maintaining means, saidshift-responsive means comprising means for maintaining a pair ofelectron beams extending along opposite sides of said crucible, inpositions to be affected by the space charge of said first-mentionedbeam on a lateral shift of said impingence area from its normalposition.

References Cited by the Examiner UNITED STATES PATENTS 2,640,948 6/ 1953Burrill.

3,046,936 7/1962 Simons.

3,112,391 11/1963 Sciaky 219-124 3,146,335 8/ 1964 Samuelson.

3,152,238 10/1964 Anderson 219-121 RICHARD M. WOOD, Primary Examiner.

JOSEPH V. TRUHE, SR., Examiner.

